Typically, to transfer a file over the Internet for viewing at a client, the client sends a download request to a server providing the file content, the server responds with the file data, and then the client displays the received file contents locally. When a server receives a request for content from a user, the server will locate the file and start streaming it to the client, without regard to what content the client already has downloaded previously. As a result, this typical data transfer regimen does not account for whether the user already has a portion of this file on their local device and can lead to inefficiencies and redundancies.
For instance, a user may fetch email from a corporate server via an IMAP or POP3 protocol. Then, the same user may go to the company's website for mail lists and see the archives of email on a particular list. The user may then download the email content again, but this time from a different application via a different communication mechanism.
In another example, a user may download a presentation locally from a server on the Internet. If the user wants to view the file again from the Internet, he or she may return to the same location the presentation was obtained from and select the same link used to previously obtain the presentation. If a different server is now serving that download request, the presentation file will be re-downloaded, instead of using the previously-downloaded presentation file from the cache of the user.
In yet another example, a user may download his or her email via an email application on the client. If the network connection drops while a large attachment is being downloaded, the current email application is configured to re-download the attachment from the start on a next email fetch. This re-download may be expensive for the user (if using something like mobile broadband, where connections are flaky). In addition, if the user tries to download the attachment via a web interface to his or her mailbox, the entire file will be downloaded, irrespective of whether the whole file is already downloaded via the email application.